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Sao (/ˈs./ SAY-oh; Greek: Σαώ) is a prograde irregular satellite of Neptune. It was discovered by Matthew J. Holman et al. on August 14, 2002.[5]

Discovery [1][2]
Discovered by
DiscoveredAugust 14, 2002
Mean orbital elements [3]
Epoch June 10, 2003
Semi-major axis22.228 Gm
Inclination53.483° *
Orbital period2912.72 days
(7.97 yr)
Physical characteristics
Mean diameter44 km[4] **
Albedo0.04 assumed[4]
Irregular satellites of Neptune.

Sao orbits Neptune at a distance of about 22.4 million km and is about 44 kilometers in diameter (assuming an albedo of 0.04).[4]

Sao follows an exceptionally inclined and moderately eccentric orbit illustrated on the diagram in relation to other irregular satellites of Neptune. The satellites above the horizontal axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity.

The satellite is in Kozai resonance, i.e. its inclination and eccentricity are coupled (the inclination of the orbit decreases while eccentricity increases and vice versa).[5]

Sao, or Neptune XI, like many of the outer satellites of Neptune, is named after one of the Nereids; Sao was associated with sailing and is referred to as "The rescuer" or "Safety". Before the announcement of its name on February 3, 2007 (IAUC 8802), Sao was known provisionally as S/2002 N 2.


  1. ^ JPL (2011-07-21). "Planetary Satellite Discovery Circumstances". Jet Propulsion Laboratory. Retrieved 2011-10-24.
  2. ^ Green, Daniel W. E. (January 13, 2003). "Satellites of Neptune". IAU Circular. 8047. Retrieved 2011-10-24.
  3. ^ Jacobson, R. A. (2008). "NEP078 – JPL satellite ephemeris". Planetary Satellite Mean Orbital Parameters. Retrieved 2009-09-23.
  4. ^ a b c Sheppard, Scott S.; Jewitt, David C.; Kleyna, Jan (2006). "A Survey for "Normal" Irregular Satellites around Neptune: Limits to Completeness". The Astronomical Journal. 132: 171–176. arXiv:astro-ph/0604552. Bibcode:2006AJ....132..171S. doi:10.1086/504799.
  5. ^ a b Holman, M. J.; Kavelaars, J. J.; Grav, T.; et al. (2004). "Discovery of five irregular moons of Neptune" (PDF). Nature. 430 (7002): 865–867. Bibcode:2004Natur.430..865H. doi:10.1038/nature02832. PMID 15318214. Retrieved 24 October 2011.

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